1. Field
Embodiments of the present disclosure are generally directed to power generation and, in particular, to batteries and fuel cells which generate electrical power from carbohydrates.
2. Description of the Related Art
Recent surging fossil fuel costs have heightened the awareness to adopt a more diversified energy policy to ensure energy security and reduce the addiction to fossil fuels. Possible solutions being pursued include biofuels, distributed energy resources (DER), and distributed generation that utilizes a variety of readily available fuels and natural resources. Such distributed power generation must also have an efficient storage mechanism to reduce loss and to increase grid stability, efficiency, and readiness.
Biological systems commonly use carbohydrates as a storage medium and energy source. Mimicking such a natural pathway for future energy storage and conversion would seem to be extremely attractive. For example, it seems natural that carbohydrate fuels would be the most environmentally friendly. Furthermore, food crops rich in carbohydrates (e.g. sugars) are likely to remain available and plentiful in the foreseeable future throughout the world. The production of sugars could easily be scaled up world-wide without technical or social barriers. Additionally, producing sugars as a fuel differs from biofuel production in that a simple extraction would suffice without a complicated energy-intensive refinement process.
As attractive as it may be to mimic nature's chemical energy storage and conversion from sugars to thermal mechanical power, it is even more impressive to convert the stored energy directly to electrical power with oxygen in the air in a battery or fuel cell configuration.
In biological metabolism processes, harnessing carbohydrate energy relies upon enzymes. In past decades, there have been a number of attempts to obtain electric power from carbohydrates in battery or fuel cell configurations and two general classifications of battery or fuel cell designs have been reported, distinguished by the type of catalyst used. One classification of battery or fuel cell designs, biotic designs, attempt to mimic nature by using catalysts such as enzymes or directly using microorganisms. Biotic designs have been considered for commercialization for biomedical applications, such as implants. The other classification of battery or fuel cell designs, abiotic designs, utilizes inorganic catalysts, or precious metals, primarily for glucose sensing and medical implant applications.
Prior pursuits in abiotic fuel cell designs using carbohydrate fuels have been hampered by the inability of the precious metal-based catalysts to sustain power production due to poisoning. The precious metal catalysts are also cost-prohibitive. Recent work on abiotic design report abiotically catalyzed glucose fuel cells for implants that can generate about 1-3 μW/cm2 at about 37° C. Regarding microbial designs, an example that is often cited, operated out of microbial consortia with self mediated electron transfer, can generate about 0.431 mW/cm2 (at about 664 mV).
In view of this, biotic designs have been favored, to date, using either microbial or enzymatic catalysts. Enzymatic biotic designs demonstrating about 0.28 mW/cm2 (+0.88 V), while operating at about 37° C. at about pH 5 and about 5 mM glucose with glucose oxidase Penicillium pinophilum have been reported. Improved glucose-oxygen enzymatic cell operation achieving about 1.45±0.24 mW/cm2 at about 0.3 V has also been reported.
However, low power density, short lifetime, and complex electrode design still impede the progress in most of these biotic systems. Furthermore, delicate engineering practices are required to achieve workable devices. To separate the fuel and the oxygen, most abiotic and biotic devices use membranes to achieve better efficiency and sustainability. To increase power density in abiotic systems often requires careful optimization and processing in placing the nano-size catalysts on carbon support. Such designs always need to juggle the trade-offs between the costs associated with catalysts and cell performance. They also have to control the balance of the plant to maintain suitable operative conditions and to prolong the life of the catalysts.
Similarly, biotic systems also have to cultivate the biological species with delicate control of the immediate environment to which the species are exposed. In a direct electron transfer scheme, the architecture of immobilizing the biocatalysts onto a sustainable support with an effective configuration remains a difficult challenge. In a mediated system, the complexity of involving multiple species in the electron transfer, from the fuel to enzyme cofactor, to the mediator, and to the current collecting electrode, coupled with the sensitivity of the biocatalyst, inherently results in a less efficient and delicate pathway.